1. Field of the Invention
This invention relates to charged particle beams such as focused ion beams and electron beams that move across a target surface, and more particularly to apparatus and methods for controlling the beam movement.
2. Description of the Related Art
Charged particle beams such as focused ion beams (FIBs) and electron beams may be used for implantation into a substrate, scanning a target to locate a fiducial mark, and in the case of ion beams dislodging material from a target surface such as by sputtering. While the remainder of this discussion will concentrate on FIBs, the invention is also applicable to other charged particle beams that are scanned across a surface.
A FIB column focuses an ion beam to a fine point and directs it to a precise region on a surface, usually for the purpose of implanting beam ions into the surface. The doping produced with a FIB is determined by the type of dopant ions used. The ion implant density is controlled by the ion current density in the beam and the dwell time on the substrate, while the depth of implant is controlled by the beam voltage (energy). Accurately defined lines, rectangles and trapezoids can be formed by scanning the beam across successive adjacent lines to fill a desired area. A representative ion beam column is disclosed in V. Wang, J. W. Ward and R. L. Seliger, "A Mass Separating Focused Ion Beam System for Maskless Ion Implantation", Journal of Vacuum Science and Technology, Nov.-Dec. 1981, pages 1158-63.
The primary purpose of using a FIB is to eliminate the conventional masking steps, and thereby increase the accuracy of implanting ions into a semiconductor. U.S. Pat. No. 4,563,587 issued Jan. 7, 1986 to Ward, et al. and assigned to Hughes Aircraft Company, the assignee of the present invention, discloses a focused ion beam column for implanting ions into semiconductor wafers. An ion beam is obtained from an ion source that typically emits several species of ions. One type of ion is selected and the voltage of the column's mass separator is set so that unwanted ions are deflected. The remaining chosen ions are accelerated through the column and directed onto the target wafer.
While offering significant advantages over the use of masks for implantation, current FIB systems are also somewhat limited. An octopole deflector is normally used to scan the beam, in a straight line and at a constant speed, over the target surface. If a greater implant density is desired, the scanning speed can be slowed and/or the beam can perform multiple scans over the same line. If a doping gradient is desired for a particular line, multiple lines of progressively shorter length are superimposed over each other. This is a time consuming process, requiring an adjustment after each line segment, and significantly reduces throughput.
Three-dimensional rectangle and trapezoid patterns can be developed by scanning a series of adjacent lines. If a doping gradient is desired from line to line, multiple scans can be superimposed over desired lines, with the number of scans increasing or decreasing for successive lines in accordance with the desired gradient. Again, this is a time consuming process, and the number of available geometric patterns is quite limited.
A variety of inaccuracies are also inherent in current systems. For example, a finite amount of time is required to process the control signals and for the beam to travel between the mass separator and the target. This delay can result in the beam particles not reaching the target until scanning has already begun, and in scanning beyond the intended termination of a line. Another example involves the control of stigmation, which is the degree of roundness or circularity of the beam. The beam is generally somewhat elliptical when it enters the octopole deflector, and a voltage pattern is applied to the deflector plates to make it more round. Only a limited correction is currently available to account for specific angles between the beam's major elliptical axis and the octopole axis, such as 45.degree. or 90.degree., but not the angles in between.
Another example of a built-in inaccuracy is in the location of fiducial marks on the target surfaces. The beam is scanned across the surface to detect the leading and trailing edges of a fiducial mark. However, because of delays in the beam transit time and signal processing, the detected location of the mark may be inaccurate.
Another problem is that there is generally some misalignment between the x-y coordinate systems for the octopole deflector and the target wafer. To adapt the beam deflection to the wafer position, the beam's coordinate system must be transformed to match that of the wafer. This is currently performed with analog coordinate transformation circuitry located just prior to the octopole synthesis circuitry used to transform the x, y deflection commands into the eight octopole plate signals. This in effect adds more stages to the analog synthesis circuitry, reducing the system bandwidth, increasing the system noise, and in general slowing it down.